1. Field of the Invention
The invention relates to a method for detecting acoustic waves in a composite material using fiber optic sensors. The invention also relates to a novel sensor and system for detecting the acoustic waves.
2. Description of Prior Art
It is known in the prior art to embed sensors in composite materials to determine internal stresses and breakages of the fibers or layers forming the composite materials. U.S. Pat. No. 3,350,926 teaches stress transducers 10 which are embedded in materials (see FIGS. 4 and 5) so that they may detect internal stresses. The transducers are semiconductive piezoresistive elements mounted in housings. U.S. Pat. No. 3,779,071 teaches the use of conductive fibers (e.g. graphite fibers) embedded in a plied material. The resistance of the fibers is monitored to determine changes in the plied material.
U.S. Pat. No. 4,772,092 teaches the use of light fibers embedded in a laminated material. Light is coupled to the input side of the fibers and is detected at the output side of each fiber. When the material cracks, fibers adjacent to the material will also crack and break. Accordingly, light applied to the input of such broken fibers will either not reach the output or will reach the output with a far lesser intensity. Accordingly, by detecting a decrease in intensity at the light output, it is possible to determine the position of cracks in the composite material. U.S. Pat. No. 4,836,030 teaches a like method employed in aircraft skin laminated materials. Once again, a decrease in light intensity indicates cracks at the position of the fibers which show this decrease.
In U.S. Pat. No. 4,781,056, at least one optical fiber is arranged to form a network, and light is emitted into one end and received out of the other end of the network. The network is embedded in a composite material. The light intensity transmitted provides information concerning the presence of strain which is used for weight detection (a scale).
In U.S. Pat. No. 4,603,252, optical fibers are embedded in laminated materials in regular patterns, and light transmitted through the fibers is monitored for changes in the patterns and for cracks. In U.S. Pat. No. 4,636,638, a strain responsive device includes an optical fiber 20, and spaced strain responsive devices 22 bonded to the optical fiber.
In the above systems, a break in the composite material will be detected only if a fiber is present at one location of the break, and the fiber is also broken. Accordingly, the fibers must be arranged in a grid throughout the monitored material (as shown, for example, in the '252 and '056 patents). To provide adequate monitoring, the spacing between fibers has to be of the order of 1 inch, i.e. a very dense grid is needed.
With respect to the patents using optical fibers, it is noted that they detect only cracks and breakages, and they provide information only when the optical fiber is broken.
It is also known to detect acoustic waves in water with fiber optic sensors. U.S. Pat. No. 4,751,690 teaches a fiber optic interferometric hydrofoam which uses optical fibers on the top and bottom of bending beams. The top and bottom fibers form, in effect, the two arms of an interferometer. In an embodiment illustrated in FIG. 6 of the patent, the free ends of the optical fibers are mirrored.
It is also known in the art to use elliptical core fibers for interferometric measurements. The two spatial modes of the elliptical core fibers are employed as the two arms of the interferometer. For example, see "Use of Highly Elliptical Core Fibers for Two-Mode Fiber Devices" by Kim et al, Optics Letters, September 1987, Vol. 12, No. 9, p 729 et seq. and "Elliptical Core Two-Mode Fiber Strain Gauge" by Blake et al, in SPIE Vol. 838, Fiber Optic and Laser Sensors V (1987), pages 332 et seq. U.S. Pat. No. 4,773,753 teaches a fiber optic sensor having circular cores and elliptical clads surrounding the cores.
It is also known that acoustic emissions are caused by the delamination and breakage of fibers in composite materials.